Dispersion of aluminium oxide

ABSTRACT

Aqueous dispersions of a pyrogenically-produced aluminum oxide are prepared form a mixture containing water, additives and a high-surface-area pyrogenically-produced aluminum oxide with a BET specific surface area of more than 115 m 2 /g and a Sears number of more than 8 ml/2 g using controlled high shearing conditions. Prior to subjecting the mixture to high shear conditions, the pH of the mixture is adjusted to a value between of 2 to 11 (preferably 3 to 8). The aqueous dispersion has a mean particle size distribution of d 50 &lt;150 nm. These dispersions are useful in the production of Inkjet media, digital imaging products and other printing media and result in high color densities, photo-realistic reproductions and particularly good print and gloss quality.

RELATED APPLICATIONS

This application claims priority to European Patent Application No. 10152 745.4 filed Oct. 25, 2001.

BACKGROUND OF THE INVENTION Technical Field

The invention relates to a dispersion of pyrogenically-producedaluminium oxide, a process for its production and its use.

DISCLOSURE OF THE INVENTION

The invention provides a dispersion, which is characterised in that itcontains a high-surface-area, pyrogenically-produced aluminium oxide,which has a BET specific surface area of more than 115 m²/g and a Searsnumber of more than 8 ml/2 g.

In one embodiment of the invention, the aluminium oxide can bechracterised in that the dibutylphthalate absorption of the powder,measured on a 16 g portion, is not measurable (no end-point detection).

The pyrogenically-produced aluminium oxide that can be used according tothe invention, can be produced by the flame oxidation, or preferablyflame hydrolysis, method, an evaporable aluminium compound, preferablythe chloride, being used as the starting material.

The content of aluminium oxide can be 25±15 wt. %.

The dispersion may contain the following additional constituents:

Organic or inorganic acids or bases, salts, additives such as ionic ornon-ionic surfactants, polymers, polyelectrolytes or biocides as well asother auxiliary substances to stabilise the dispersion or to increaseits quality or storage stability.

The dispersion according to the invention may also contain, in additionto aluminium oxide, inorganic acids (such as phosphoric acid, nitricacid, sulfuric acid, . . . ), organic acids (such as formic acid, aceticacid, propionic acid, oxalic acid, malonic acid, . . . ), inorganic ororganic bases (such as potassium hydroxide, organic amines, . . . ),salts, (such as sodium chloride, potassium formate, calcium nitrate),buffer systems (such as for example potassiumdihydrogenphosphate/phosphoric acid buffer, acetic acid/sodium acetatebuffer, . . . ) ionic, or non-ionic surfactants, polyelectrolytes,polymers or other dispersing agents or stabilisers, and also biocides.

The invention further provides a process for the production of thedispersion according to the invention, which is characterised in that ahigh-surface-area, pyrogenically-produced aluminium oxide, which has aBET specific surface area of more than 115 m²/g and a Sears number ofmore than 8 ml/2 g, is mixed with water, a pH value of 2 to 11,preferably 3 to 8, in particular 2 to 8, is set and the mixture isdispersed by the introduction of shearing forces.

The invention is formed by mixing a high-surface-area,pyrogenically-produced aluminum oxide, which has a BET specific surfacearea of more than 115 m²/g and a Sears number of more than8 ml/2 g, withwater and additives setting a pH value of 2 to 11 (preferably 3 to 8),and dispersing the mixture by the introduction of controlled shearingforces. To disperse the high-surface-area, pyrogenically-producedaluminium oxide, shearing equipment such as rotor-stator-type machines(batch- or continuous in-line machines), ball mills, pearl mills,agitated ball mills or high-energy shearing processes (in which thedispersion is dispersed under high pressure at pressures of over 1000bar) is used and the dispersions obtained have a mean particle sizedistribution of d₅₀<150 nm (d₅₀ less than 150 nm; measured by dynamiclight scattering).

The dispersion according to the invention has the following advantages:

-   -   high cationic charge on the surface of the particles, as        compared with dispersions produced from aluminium oxides (-oxide        hydroxides) by the wet chemical method    -   particle size distributions of the dispersion can be set in a        defined way, in contrast to dispersions of other pyrogenic        aluminium oxides or aluminium oxides (-oxide hydroxides)        produced by the wet chemical method, which are produced by        different dispersion processes    -   high dispersion purity, as compared with dispersions produced        with aluminium oxides (-oxide hydroxides) produced by the wet        chemical method    -   precisely-controllable electrolyte level within the dispersion,        as compared with dispersions of other pyrogenic aluminium oxides        or aluminium oxides (-oxide hydroxides) produced by the wet        chemical method, the electrolyte concentration of which cannot        be set precisely as a result of the starting powder or the        dispersion process    -   very high surface area of the pyrogenic aluminium oxide as        compared with dispersions of other pyrogenic aluminium oxides or        aluminium oxides (-oxide hydroxides) produced by the wet        chemical method    -   highly distinctive “structure/crosslinking” of the aluminium        oxide primary particles within the dispersion, as compared with        dispersions of other pyrogenic aluminium oxides or aluminium        oxides (-oxide hydroxides) produced by the wet chemical method    -   a high degree of hardness and abrasion resistance of the        particles distributed in the dispersion, as compared with        dispersions of aluminium oxides (-oxide hydroxides) produced by        the wet chemical method, in which the primary particles are less        hard.

The dispersion according to the invention is eminently suitable forfurther processing in Inkjet coating pigments. The invention furtherrelates to a process for the production of Inkjet coating pigments thathave been produced with the dispersions according to the invention andthe glossy media produced from them. These media are characterised by avery high gloss value, very high colour densities when printing, largecolour space, extremely low drying times, a high absorption capacity and(in comparison with other aluminium oxides) increased water resistance,all of which are desirable characteristics.

The following examples demonstrate that the dispersion according to theinvention is superior to known dispersions with regard to itsprocessability in a coating pigment and in the associated Inkjet medium.

The recipes used for Inkjet coating pigments are guide recipes. Thusbinders other than polyvinylalcohol (with various saponification grades)can be used in combination or alone, such as for examplepolyvinylpyrrolidone, polyvinylacetate, silanised polyvinylalcohols,styrene-acrylate lattices, styrene-butadiene lattices, melamine resins,ethylene-vinylacetate copolymers, vinylpyrrolidone-dimethylaminopropylenemethacrylamide copolymers, polyurethane resins,synthetic resins such as polymethylmethacrylates, polyester resins (forexample unsaturated polyester resins), polyacrylates, modified starch,casein, gelatines, cellulose derivatives (for examplecarboxymethylcellulose) and also in other quantity ratios to thepyrogenic aluminium oxide than those stated in the application example.

The coating according to the invention may also contain, in addition tothe dispersion according to the invention, one or more other pigments,and/or dispersions. These may be: calcium carbonates, layered silicates,aluminium silicates, plastic pigments (for example polystyrene,polyethylene, polypropylene), silicas (for example colloidal silicas,precipitated silicas, silica gels, pyrogenic silicas, pyrogenic mixedoxides), cationised variants of the stated silica compounds, aluminiumcompounds (for example aluminium sols, colloidal aluminium oxides andtheir hydroxy compounds, such as pseudo-boehmite, boehmite, aluminiumhydroxide), magnesium oxide, zinc oxide, zirconium oxide, magnesiumcarbonate, kaolin, clay, talc, calcium sulfate, zinc carbonate, satinwhite, lithopones, zeolites or dispersions of these compounds.

Both the solid content of the dispersion and also that of the coatingpigment can be varied in comparison with the application examples.Further possible variations consist in changing the pH value, thedispersion method, the composition of the coating pigment formulationand the dispersion medium.

To increase the water-resistance of the binder system and thus thecoating, crosslinkers may be used, such as: zirconium oxides, boricacid, melamine resins, glyoxal and isocyanates and other molecules thatbind the molecule chains of the binder system with each other.

Furthermore, auxiliary agents such as optical brighteners, de-foamingagents, wetting agents, pH buffers, UV absorbers and viscosity improverscan also be used.

Examples of cationic substances for the cationising of the binder are:polyethyleneimine, polyallylamines, polyvinylamines, polymers consistingof cationic monomers such as primary, secondary and tertiary amines,which are located on the side-chains, and quarternary ammoniumcompounds.

The coating structure of the medium may also differ from the applicationexamples, in that the coating may consist of several coating layers ofthe same type or other layers, and the coating that has been producedwith the dispersion according to the invention is only located in one ofthe layers present. Thus, for example, further ink-absorptive coatings(for example precipitated silica coatings) may be applied under theInkjet coating according to the invention. Furthermore, one or morepolymer layers (e.g. polyethylene) can be applied to the substrateand/or to the coating according to the invention, in order to increasethe mechanical stability and/or gloss of the coating (for examplephotographic base paper, lamination). Even if the Inkjet medium containsone or more of the layers described above, and is provided with thecoating according to the invention, printability is improved for themajority of printing applications.

The coating pigment recipe can be applied to a substrate by allconventional application processes (alone or in combination).Conventional processes (which are not, however, restricted to the statedprocesses) are: rolling, blade application, airbrushing, doctor blade(profiled, smooth, split), the cast-coating process, film pressing,bonding-pressing, blade coating, curtain-coating and slot-dieapplication (for example coating blade). The processes allowingextremely homogeneous coating, for example cast-coating, curtain-coatingand slot-die application, are preferred.

The coated substrate can be dried by all of the conventional processes(alone or in combination). Conventional processes (which, however, arenot restricted to the stated processes) are: air- or convection drying(for example hot air channel), contact- or conduction drying, energyradiation drying (for example infra-red and microwave).

The invention further provides coatings, which have an aluminiumoxide-binder weight ratio of 8:1 to 1:2. The coatings can be used forthe production of Inkjet-media, digital imaging products and otherprinting media.

The invention further provides recording media consisting of aluminiumoxide dispersions, which are characterised in that they consist ofsubstrates with glossy coatings, in which the coatings contain VP Alu130 dispersions and dispersion additives (such as acids, bases,surfactants, polymers, polyelectrolytes etc.) and a binder system, thealuminium oxide particles that build up the coating originating from adispersion according to the invention, with an average particle size ofd₅₀<200 nm (d₅₀ less than 200 nm; measured by dynamic light scattering).

The aluminium oxide dispersion can be used in coatings for materialssuch as paper, metals, glass, ceramics, plastics, polymers, rubbers andtextiles. Furthermore, it can be used to polish or clean metals,semi-conductor elements in the electronics industry, glass, ceramics andother hard materials.

The coatings that have been produced from coating pigments, may have asolid matter content of 15%, and preferably of 18% and more.

The coatings may have a substrate with a substance weight of 80 to 250g/m². The substrate may have a Cobb₁₀value of 0 to 60.

A water-impermeable sealing coat may be applied to the substrate. Thesubstrate may be a polymer film.

The coatings may be used for the production of Inkjet media (such aspaper, films, textiles, . . . ), digital imaging products and otherprinting media.

The dispersions according to the invention may be used for the coatingof fluorescent tubes, lightbulbs or other light sources.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows VP W 520 “F” shearing tests measured on Jan. 11, 2001.

FIG. 2 shows VP W 520 “F” shearing tests measured on Jan. 25, 2001.

FIG. 3 shows changes in the viscosity of W 520 “F” with increasingshearing time.

DETAILED DESCRIPTION OF THE INVENTION EXAMPLE 1 Production of theAluminium Oxide that can be used According to the Invention

320 kg/h previously-evaporated aluminium trichloride (AlCl₃) is burnedtogether with 100 Nm³/h hydrogen and 450 Nm³/h air in a burner of knownconstruction.

After the flame reaction, the fine-particle, high-surface-area aluminiumoxide is separated from the hydrochloric acid gases that have alsoformed, in a filter or cyclone, any HCl traces still adhering being thenremoved by treatment with moistened air at increased temperature.

The high-surface-area pyrogenic aluminium oxide formed has thephysical/chemical reference data shown in Table 1. The data for thepyrogenic aluminium oxide that can be obtained commercially fromDegussa-Hüls AG/Frankfurt (commercial name Aluminiumoxid C) are alsoincluded in Table 1 for reference.

TABLE 1 High-surface-area aluminium oxide according to the inventionAluminium Unit Alu 130 oxide C BET m²/g 121 100 Specific Surface areaSears number ml/2 g 9.38 7.05 (pH 4 to 9) PH 4% aqueous 4.93 4.5dispersion Drying loss wt. % 3.3 3.0 Bulk density g/l 55 48 Tampeddensity g/l 63 57 DBP-Absorption wt. % Not measurable, 231 no detectableend point. DBP: Dibutylphthalate

The measurement of the Sears number is disclosed in EP 0 717 008.

EXAMPLE 2 Production of the Dispersion According to the Invention

First, 280 litres de-ionised water are brought to pH 3.9 with apropionic acid in a receiving vessel. 80 kg of the powder described inexample 1 with a surface area greater than 115 m2/ g (equivalent to 20wt.-% aluminium oxide) are then introduced into the water with arotor-stator machine. After incorporating the whole quantity of thepowder, the suspension obtained is intensively sheared for ca. 60minutes.

Whilst the powder is being introduced, the pH value is maintained atpH=4.0 to 4.1 by adding 18 litres of semi-concentrated acid. Afterintake of the aluminium oxide and immediately before shearing, a sampleof the suspension is taken and a d₅₀ value of d₅₀=160 nm is determined(measurement in dilution by means of dynamic light scattering). Thesubsequent shearing process is carried out with the rotor-stator machineat maximum shear energy and lasts for a total of 60 minutes.

After completion of the shearing process, the pH value is 4.1. Afteradding 2 kg of a biocide, the pH value was brought to the final pH of3.9 with a further 6 litres of propionic acid. Once production of thedispersion is complete, the liquid volume is increased to 400 litres byadding 14 litres distilled water. The particle size set after theshearing process is d₅₀=130 nm.

Properties of the dispersion according to example 2 of the invention (VPW 520). During production of the dispersion the average particle sizefalls significantly as the shearing time continues (cf. FIG. 1): withoutshearing (0 min) the value is still d₅₀=160 nm, but after 60 minutes'shearing it falls to d₅₀=130 nm:

TABLE 2 Comparison of the samples of the dispersion according to theinvention immediately after production and two weeks later 0 min 15 min27 min 30 min 45 min 60 min Shear. Shear. Shear. Shear. Shear. Shear.d₅₀ in (nm) 160 155 149 143 137 130 (day of production) d₅₀ in (nm) 170155 153 147 139 136 (control, 2 weeks)

During production of the dispersion according to the invention, the pHvalue does not depend on the shearing time (cf. Table 2). The change inpH within two weeks is also the same for all samples:

TABLE 3 pH-values of the samples of the dispersion according to theinvention (dependency on shearing and time) 0 min 9 min 15 min 30 min 45min 60 min Shear. Shear. Shear. Shear. Shear. Shear. pH-value 3.9 4.04.0 4.1 4.0 3.9 (production) pH-value 4.5 4.5 4.6 4.6 4.5 4.5 (control,2 Weeks)

The gloss values are measured with a Byk-Gardner gloss meter using testcard 2855 (black spectrum) as a basis.

The printing properties of the coating are evaluated by printing out atest image on the coating, using an HP 550 C printer and an Epson StylusColour 800 printer respectively and having these printed coatingsevaluated by 3 independent persons on the basis of the attached scale ofmarks.

The colour densities are measured on the basis of the test image, whichalso contains full-area colours (black, magenta, cyan, yellow) using aGretagMacbeth (trademark) SpectroEye at an observation angle of 2° and aD50 light source.

EXAMPLE 3 Production of an Inkjet Coating

-   -   Educts: aluminium oxide dispersion according to example 2        polyvinyl alcohol Mowiol 40-88 (Clariant, D) polyester film

Recipe and method for the production of Inkjet coating pigments:

The aqueous polyvinyl alcohol solution (Mowiol 40-88, Clariant) with12.14% solid content, is placed in a 400 ml beaker and a quantity ofwater is added to it, so that, together with the aluminium oxidedispersions according to example 2, a solid content of 18% is obtained.The relevant dispersion is slowly dropped into the polyvinyl alcoholsolution using a pipette, within 5 minutes, whilst stirring at 500 rpm.Once it has been added, stirring continues for a further 30 minutes at500 rpm to obtain a homogenous coating pigment. The coating pigments arethen de-aerated using a dessicator and water-jet pump. As a control, theactual solid matter, pH value and viscosity are measured after mixingthe coating pigments.

The parts in the table below are understood to mean parts by weight inrelation to the solid matter.

TABLE 4 Coating pigment reference values Aluminium oxide 100 partsdispersion, acc. to example 2, 15 minutes' dispersion time Aluminiumoxide 100 parts dispersion, acc. to example 2, 30 minutes' dispersiontime Aluminium oxide 100 parts dispersion, acc. to example 2, 60minutes' dispersion time PVA 40-88  20 parts  20 parts  20 parts Actualsolid 18.03 18.01 18.00 content of the coating pigments in % PH value4.7 4.6 4.6 Viscosity, Brookfield in mPa s after 24 h After  5 rpm 920824 748 stir-  10 rpm 840 716 664 ring  20 rpm 715 608 534  50 rpm 560512 462 100 rpm 463 398 346 Comments on good good good production ofcoating pigments

The viscosity data obtained show that the dispersion according toexample 2, and the longest dispersion time, produce the lowestviscosities. This is desirable as the solid matter in the coatingpigment can still be increased without obtaining viscosities that aretoo high to be applied.

Method for the production of an Inkjet coating from the Inkjet coatingpigment referred to above

A 100 micron thick, un-treated polyester film is coated using anErichsen Film Applicator device with a 120 micron wet film spiralapplicator. The coating pigment applied is dried using a hot air dryer.

Data for the Coatings Obtained:

Aluminium oxide 100 parts dispersion acc. To example 2, 15 minutes'dispersion time Aluminium oxide 100 parts dispersion acc. to example 2,30 minutes' dispersion time Aluminium oxide 100 parts dispersion acc. toexample 2, 60 minutes' dispersion time PVA 40-88  20 parts  20 parts  20parts Application behaviour good good good Average application 26.1 25.926.2 weight in g/m² and s = 0.4 s = 0.2 s = 0.3 standard deviation*Coating properties Aluminium oxide 100 parts dispersion acc. to example2, 15 minutes' dispersion time Aluminium oxide 100 parts dispersion acc.to example 2, 30 minutes' dispersion time Aluminium oxide 100 partsdispersion acc. to example 2, 60 minutes' dispersion time PVA 40-88  20parts  20 parts  20 parts Adhesion, surface Good Very good Very good andsmoothness of adhesion to adhesion to adhesion to the coating the film,the film, the film, homogeneous homogeneous homogeneous surface surfacesurface Gloss at 20°/ 24.6 26.3 28.8 standard s = 0.4 s = 0.1 s = 0.3deviation at n = 5 Gloss at 60°/ 46.3 49.7 54.7 standard s = 0.2 s = 0.3s = 0.3 deviation at n = 5 *Note: the printing test is carried out withcoatings of virtually the same coating weight.

Discussion of the Gloss Values

It can clearly be seen from the gloss values, that the dispersionaccording to the invention produces higher gloss values in the Inkjetcoating after 60 minutes' dispersion time, than the dispersions producedby other methods. With photo-realistic coatings, a high gloss isdesirable, as already disclosed in EP 0 732 219. The gloss values arelower than those in EP 0 732 219, but this is due to the differentprocesses for the production of the Inkjet medium and not to the coatingpigment. A spiral applicator is consiously used in this test todetermine the contribution of the dispersion in the Inkjet coating tothe gloss. With the cast-coating process in EP 0 732 219, the gloss isprimarily determined by the process itself.

Printing test (determined by 3 independent persons) Top mark: 0.75bottom mark: 6

Aluminium Aluminium Aluminium oxide oxide oxide dispersion dispersiondispersion acc. to acc. to acc. to example 2, example 2, example 2, 15minutes' 30 minutes' 60 minutes' dispersion dispersion dispersionDispersion used time time time Four colour print Hewlett-Packard 550 CColour Magenta/ 1.25 1 1 intensity Yellow/Cyan Black 1 1 1 dot Black in1.5 1.25 1.25 sharpness colour transitions Colour to 1 1 1 colour Blackto 1.25 1 1 colour Contours 1 1 1 Print 1.25 1.25 1 Halftone 1 1 1 Photo1.25 1.25 1 quality Total 10.5 9.75 9.25 evaluation Average 1.17 1.081.03 marks Four colour print Epson Stylus Color 800 Colour Magenta/ 1.251 0.75 intensity Yellow/Cyan Black 1 1 0.75 Dot Black in 1.25 1 1sharpness colour Transitions Colour to 1 1 1 colour Black to 1 1 1colour Contours 1 1 1 Print 1 1 1 Halftone 1.25 1 1 Photo quality 1 10.75 Total 9.75 9 8.25 evaluation Average 1.08 1.00 0.92 mark

When using two different types of printer, the coating containing thedispersion with the longest dispersion time, according to example 2,produces the best results.

Colour densities Aluminium Aluminium Aluminium oxide oxide oxidedispersion dispersion dispersion acc. to acc. to acc. to example 2,example 2, example 2, 15 minutes' 30 minutes' 60 minutes' Dispersiondispersion dispersion dispersion used time time time DIN HP 550 C PapPol Filter selected Paper white 0 0 0 Mean black 2.33 2.36 2.42 Meanyellow 1.67 1.69 1.73 Mean cyan 2.33 2.35 2.39 Mean magenta 1.39 1.421.46 Total 7.72 7.82 8 Epson Stylus Color 800 Pap Pol Filter selectedMean black 3.07 3.08 3.14 Mean yellow 2.15 2.17 2.21 Mean cyan 3.23 3.253.33 Mean magenta 2.34 2.35 2.42 Total 10.79 10.85 11.1

When examining the colour densities, it can be seen that the coatingcontaining the dispersion according to example 2 of the invention, withthe longest dispersion time, reproduces the highest colour densities.This is desirable to obtain the most photo-realistic reproductionpossible.

EXAMPLE 4 Production of an Inkjet Coating

-   -   Educts: aluminium oxide dispersion according to example 2        polyvinyl alcohol Mowiol 40-88 (Clariant)

Recipe and method for the production of Inkjet coating pigments:

The aqueous polyvinyl alcohol solution (Mowiol 40-88, Clariant)containing 12.14% solid matter is placed into a 400 ml beaker and aquantity of water is added so that, together with the aluminium oxidedispersions according to example 2, a solid content of 18% is obtained.The relevant dispersion is slowly dropped into the polyvinyl alcoholsolution using a pipette, within 5 minutes, whilst stirring at 500 rpm.Once it has been added, stirring continues for a further 30 minutes at500 rpm, to obtain a homogeneous coating pigment. The coating pigmentsare then de-aerated using a dessicator and a water jet pump. As acontrol, the actual solid matter, pH value and viscosity are determinedafter mixing the coating pigments.

The parts in the table below are understood to mean parts by weight inrelation to the solid matter.

Coating Pigment Reference Values

Aluminium oxide dispersion 100 parts acc. to example 2, 15 minutes'dispersion time Aluminium oxide dispersion 100 parts acc. to example 2,30 minutes' dispersion time Aluminium oxide dispersion 100 parts acc. toexample 2, 60 minutes' dispersion time PVA 40-88  18 parts  18 parts  18parts Actual solid content of 18.01 18.04 18.02 the coating pigment in %pH-value 4.7 4.6 4.6 Viscosity, Brookfield in mPa s after 24 h  5 rpm912 813 748  10 rpm 846 697 664  20 rpm 702 589 534  50 rpm 535 496 462100 rpm 443 378 346 Comments on production of good good good coatingpigments

The viscosity data obtained show that the lowest viscosities areobtained with the dispersion according to example 2 and the longestdispersion time. This is desirable, as the solid matter in the coatingpigment can be increased still further without obtaining viscositiesthat are too high for application.

Method for the production of an Inkjet coating from the Inkjet coatingpigment referred to above.

Matt-coated 110 g/M² Inkjet paper (Zweckform no. 2576) is coated usingan Erichsen Film Applicator device with a 60 micron wet film spiralapplicator. The coating pigment applied is dried with a hot air dryer.

The coatings are then calendered three times at 10 bar pressure and 50°C. with a Gradek Oy (Trademark) laboratory calender.

Data for the Coatings Obtained

Aluminium oxide 100 parts dispersion acc. to example 2, 15 minutes'dispersion time Aluminium oxide 100 parts dispersion acc. to example 2,30 minutes' dispersion time Aluminium oxide 100 parts dispersion acc. toexample 2, 60 minutes' dispersion time PVA 40-88  18 parts  18 parts  18parts Application behaviour good good good Average coating weight 12.011.9 12.1 in g/m² and standard s = 0.4 s = 0.2 s = 0.3 deviation* *Note:The printing tests were carried out with coatings of virtually the samecoating weight.

Coating properties Aluminium oxide 100 parts dispersion acc. to example2, 15 minutes' dispersion time Aluminium oxide 100 parts dispersion acc.to example 2, 30 minutes' dispersion time Aluminium oxide 100 partsdispersion acc. to example 2, 60 minutes' dispersion time PVA 40-88  18parts  18 parts  18 parts Adhesion, surface and Very good Very good Verygood smoothness of the adhesion to adhesion to adhesion to coating thepaper, the paper, the paper, homogeneous homogeneous homogeneous surfacesurface surface Gloss at 20°/  5.4  5.9  6.7 standard deviation at s =0.4 s = 0.1 s = 0.3 n = 5 Gloss at 60°/ 24.6 25.7 26.2 standarddeviation at s = 0.2 s = 0.3 s = 0.3 n = 5

Discussion of the Gloss Values

The gloss values clearly show that, after 60 minutes' dispersion time,the dispersion according to the invention produces higher gloss valuesin the Inkjet coating than dispersions produced by other methods. Forphoto-realistic coatings, a high gloss is desirable as already disclosedin EP 0 732 219. The gloss values are lower than those in EP 0 732 219,but this is due to the different processes for the production of theInkjet medium and not to the coating pigment. A spiral applicator isconsciously used in this test, to determine the contribution of thedispersion in the Inkjet coating to the gloss. With the cast-coatingprocess used in EP 0 732 219, the gloss is primarily determined by theprocess itself.

Printing test (determined by 3 independent persons) Top mark: 0.75Bottom mark: 6

Aluminium Aluminium Aluminium oxide oxide oxide dispersion dispersiondispersion acc. to acc. to acc. to example 2, example 2, example 2, 15minutes' 30 minutes' 60 minutes' dispersion dispersion dispersionDispersion used time time time Four colour print Hewlett-Packard 550 CColour Magenta/ 1.25 1.25 1 intensity Yellow/Cyan Black 1.25 1.25 1 DotBlack in 1.5 1.25 1.25 sharpness colour Transitions Colour to 1 1 1colour Black to 1 1 1 colour Contours 1 1 1 Print 1.25 1.25 1 Halftone1.25 1 1 Photo 1.5 1.25 1.25 quality Total 11 10.25 9.5 evaluationAverage 1.22 1.08 1.06 mark Four colour print Epson Stylus Color 800Colour Magenta/ 1.5 1.25 1 intensity Yellow/Cyan Black 1 1 1 Dot Blackin 1.5 1.25 1.25 sharpness colour Transitions Colour to 1.25 1.25 1colour Black to 1 1 1 colour Contours 1 1 1 Print 1 1 1 Halftone 1.25 11 Photo 1.5 1.25 1.25 quality Total 11 10 9.5 evaluation Average 1.221.11 1.06 mark

When using two different types of printer, the coating containing thedispersion with the longest dispersion time according to example 2,produces the best results.

Colour densities Aluminium Aluminium Aluminium oxide oxide oxidedispersion dispersion dispersion acc. to acc. to acc. to example 2,example 2, example 2, 15 minutes' 30 minutes' 60 minutes' Dispersiondispersion dispersion dispersion used time time time DIN HP 550 C PapPol Filter selected Paper white 0 0 0 Mean black 2.05 2.10 2.13 Meanyellow 1.49 1.54 1.58 Mean cyan 2.04 2.07 2.11 Mean magenta 1.28 1.321.35 Total 6.86 7.03 7.17 DIN Epson Stylus Color 800 Pap Pol Filterselected Mean black 2.89 2.96 2.98 Mean yellow 1.92 1.97 2.04 Mean cyan2.96 3.03 3.09 Mean magenta 2.13 2.25 2.31 Total 9.9 10.21 10.42

When examining the colour densities, it can be seen that the coatingcontaining the dispersion according to example 2 of the invention withthe longest dispersion time reproduces the highest colour densities.This is desirable to obtain the most photo-realistic reporoductionpossible.

The dispersion according to the invention is eminently suitable for theproduction of Inkjet coating pigments and their further processing toproduce high-gloss printing media, as can be seen from the examplesgiven. The Inkjet media produced in this way have a particularly goodprint and gloss quality.

1. A process for the production of aluminium oxide dispersions,comprising mixing a high-surface-area, pyrogenically-produced aluminiumoxide, having a BET specific surface area of more than 115 m²/g and aSears number of more than 8 ml/2 g, with water and one or more of thecomponents selected from the group consisting of inorganic acids,organic acids, inorganic bases, organic bases, salts, buffer systems,ionic surfactants, non-ionic surfactants, polyelectrolytes, polymers,dispersing agents, stabilizers and biocides, setting a pH value of 2 to8 with acid, and subjecting the mixture to controlled shearing forcesfor a period of about 45 minutes or more sufficient to form a dispersionhavng a mean particle size distribution of d₅₀<150 nm.
 2. The processfor the production of aluminium oxide dispersions according to claim 1,wherein the mixture is subject to controlled shearing forces in shearingequipment including rotor-stator machines, ball mills, pearl mills andagitated ball mills.
 3. The process of claim 1, wherein the meanparticle size distribution of d50 has a value that falls within therange of 139 and 130 nm.